Pneumatic artificial muscles (PAMs) are actuators that contract when pressurized with air. The most widely used PAM is the McKibben actuator, which was developed in the 1950s for actuating orthotics. McKibben actuators comprise a rubber tube enclosed in a textile mesh or braid, which contracts axially when the bladder expands it radially, acting in a manner similar to a scissor linkage and providing a load-length curve similar to that of skeletal muscle. At their ends, the bladder and mesh are crimped together to allow mechanical coupling to a load.
McKibben actuators have been used in a wide range of applications including robotics, orthotics, and industrial automation, but have not found application in direct cardiac compression (DCC), but have properties limiting the potential use of McKibben actuators inside the human body. The foremost drawback is that McKibben actuators typically have a threshold pressure of 100 kPa due to friction between the bladder and mesh coupled with an initial lack of contact between the walls of the bladder and mesh. This limitation prevents precise control of force and displacement for pressures below 100 kPa (i.e., in the operating range of cardiac compression devices). Additionally, most existing McKibben actuators have rigid, crimped attachment points at their ends that allow for easy mechanical coupling to a load. If McKibben actuators were used for DCC, such rigid end features might damage a patient's soft tissue. Further, the rigid end features have been shown to introduce local stress concentrations, causing early fatigue failure.
With regard to DCC devices, the concept of extra-cardiac assistance in the pericardial space was introduced in the 1950s, then suggested for use as a cardiac sleeve or a rubber ventricle for cardiopulmonary resuscitation in the 60s. Since then there are a number of devices are in development, both for resuscitation and chronic implant. Many have been tested in animal models, but none have FDA approval presently. The Anstadt cup has proved effective for mechanical massage of the heart to reverse cardiac arrest. The cup is elliptically shaped and covers both ventricles. By using a semi-rigid outer layer, and inflatable inner diaphragm it can deliver both diastolic extension and epicardial compression. However, the device does not synch with the native heartbeat, and therefore has potential to injure the myocardium and disturb the rhythm of the heart.
The CardioSupport system (Cardio Technologies Inc., Pine Brook, N.J.) comprises a cuff that is placed around the epicardium of the heart. The device is sealed by vacuum and contains electrodes to provide an ECG source to inflate and deflate a compressive bladder inside the cuff. Compressive force is provided by an air compressor. The Heart Booster (Abiomed, Inc., Danvers) is designed for longterm support. The compression system interfaces the heart with a cuff consisting of parallel compression tubes forming a band around the base of the heart and attached to the epicardium with surgical glue. The device uses a hydraulic drive system to fill and empty the compression tubes. The HeartPatch (Heart Assist Technologies, Australia) has non-surround cardiac assistance and independent ventricular actuation. Other strategies for DCC use electro-active polymers, ionic polymer metal nano-composites or shape memory alloys for actuation, each being limited by force generation or dynamic response. Mechanical ventricular assist devices (VADs) are also conventionally used to assist either the right (RVAD) or left (LVAD) ventricle, or both at once (BiVAD).
Passive restraint devices exist in the form of a mesh sock or girdle that surrounds the heart (e.g., CorCap Cardiac Support Device (CSD)) to provide support and reduce ventricular wall stress, such as by reducing left ventricle (LV) size and left ventricular ejection fraction (LVEF).
Regarding cardiac bench-top simulators, most existing simulators are passively actuated by external pumps or motors and do not mimic tissue material properties. One cardiac simulator that has embedded actuation in the heart wall is the Chamberlain Heart (manufactured by the Chamberlain Group, Great Barrington, Mass.). Such cardiac simulators are used, for example, in cardiothoracic training as a surgical training tool.